Working the graveyard shift increases your risk of developing cancer

by Tara Smith     Department: Health & Life Sciences

Awhile back over at Aetiology, I wrote a post about a study examining how the suppression of the hormone melatonin in nurses who worked the night shift may increase their risk of developing breast cancer. This is an idea which has been floating around in the cancer literature for a number of years, and has been supported by studies using animal models and experiments in vitro using cancer cell lines. However, human data is more difficult to interpret. Epidemiological studies have shown an elevation in risk of breast and prostate cancers in men and women who work night shifts versus those that work a more typical nine-to-five schedule, but it's difficult to prove causation from these. Is the increased risk of cancer a direct result of these working hours? Or is it a more indirect effect, due to perhaps worse eating or sleeping habits, which could result in a less robust immune response to the tumor cells, perhaps?

Though the mechanisms and details behind this increased risk remain unclear, the cancer arm of the World Health Organization has listed night shift working as a "probable" carcinogen, alongside environmental pollutants and UV rays, and the American Cancer Society says it will follow.

However, scientists suspect that overnight work is dangerous because it disrupts the circadian rhythm, the body's biological clock. The hormone melatonin, which can suppress tumor development, is normally produced at night.

If the graveyard shift theory eventually proves correct, millions of people worldwide could be affected. Experts estimate that almost 20 percent of the working population in developed countries work night shifts.

Among the first to spot the night shift-cancer connection was Richard Stevens, a cancer epidemiologist and professor at the University of Connecticut Health Center. In 1987, Stevens published a paper suggesting a link between light at night and breast cancer.

Back then, he was trying to figure out why breast cancer incidence suddenly shot up starting in the 1930s in industrialized societies, where nighttime work was considered a hallmark of progress. Most scientists were bewildered by his proposal. But in recent years, several studies have found that women working at night over many years were indeed more prone to breast cancer.

This doesn't mean the case is closed on the connection between overnight shifts and cancer; indeed, far from it. What it does, though, is validate Stevens' hypothesis, and lends a bit more credibility to investigators looking at this area of research. It also means additional attention paid to finding solutions to the problem, since millions of individuals are already living with this situation. Some strides have already been made. It's known, for example, that the greatest risk is to people who flip back and forth between day and night shifts, so employers could work to minimize that situation. Researchers are also looking into lighting that may have less of an affect on melatonin production, and Stevens advises night shift workers to just do what they can to sleep in a dark room, even if it's during the day--it appears that the light/dark balance is what really matters, whether it's during the "right" hours of the day or not.

It's a fascinating area of research, and with this new move by the WHO and ACS, one we're sure to hear more about in the coming years.

Tags: breast cancer, epidemiology, occupation, prostate cancer